For over ten years the existence of a ferroelectric phase transition in ${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_3$ has been the object of a strong controversy despite early Raman results which revealed a splitting of the soft ${\mathrm{TO}}_1$ mode at low temperature. We report here the results of an inelastic-neutron-scattering study of a 3.5 at. % Li crystal which show, for q≠0, a corresponding splitting of the TA branch between a-polarized and c-polarized phonons and no condensation of the TA mode. The phonon assignment is based on measurements made in the presence of a bias electric field. These results confirm the existence of a cubic-to-tetragonal transition in ${\mathrm{K}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_3$ even for relatively low concentrations. They reveal the order-disorder nature of this transition and the strong coupling of c-polarized TA phonons to the spontaneous polarization. A strong anisotropy of the dispersion surface is also shown to explain the asymmetric shape of the phonon peaks.

• Received 16 June 1993

DOI:https://doi.org/10.1103/PhysRevB.49.1503